Clean up config #defines.
Inline wrenObjectToValue().

Seems to give a slight speed bump. Inlining the wrenIs___
functions doesn't seem to help (may make it a bit worse.)
Track memory during deallocation correctly.

It used to subtract the number of bytes during deallocation, but
that required knowing the size of an object at free time. That
isn't always available. The old code could read a freed object
while doing this. Bad!

Instead, this tracks how much memory is still being used by
marked objects. It's correct, and is also a bit less code and
faster.